Wire current rating typically takes into account several factors including: free air rating, altitude derating and bundle derating. Wires or conductors carrying alternating current may also take into account skin and proximity effects derating.
Free air rating of a wire may be related to the surface area of the wire and not necessarily the cross-sectional area. Thus, several wires having the an equivalent cross-sectional area as a larger wire may together have a higher combined free air rating than the larger wire, because their total surface is larger. However, as the number of wires in a bundle (e.g., in a multi-wire cable) increases, the cable rating may decrease. This may be because convection with free air may be only accomplished by the wires on the outer perimeter of the bundle. Thus, a cable may exhibit bundle derating, as the number of conductors in a bundle increases. Cable rating may also decrease with increasing altitude, as free air density decreases and convection cooling decreases.
For wires carrying alternating current, skin depth is inversely related to square root of current frequency. Skin depth refers to the tendency of alternating electric current to distribute itself with greater current density near the surface of the conductor and decreasing in density with increasing depth. As the frequency increases, the skin depth decreases. This phenomenon is known as the “skin effect”. At high enough frequencies, the interior of the conductor does not carry much current, which may result in relatively high ohmic losses.
Alternating currents of the same phase and frequency in adjacent insulated conductors arranged in a bundle also have an electromagnetic effect on each other. This effect, referred to as the “proximity effect”, tends to force the currents to flow on the surfaces of the outside conductors.
The combination of skin and proximity effects may reduce the usefulness of a cable to carry high-frequency currents at high amperage.